1. Field of the Invention
The present invention relates generally to the field of flame spray methods and apparatus. Specifically, the invention relates to a combustion wire thermal spray process using wire cross sections that allow for greater application rates.
2. Description of Related Art
Combustion wire thermal spray has been in used for a number of decades to produce metallic coatings for a variety of applications. A combustion wire gun is limited in the amount of material that can be processed per unit time. The process rate depends upon the size of the gun, gas flow rates, size (diameter) of the wire, and the properties of the wire (melting point, specific heat, etc.). For most applications the process rate is sufficient to provide an economical means of coating but high volume and high speed applications have been restricted. For field or on-site work contracting applicators of thermal spray coatings set pricing rates based upon the amount of material sprayed not on the amount of time it takes which leads to incentives to increase productivity via throughput. In large scale corrosion applications on manufacturing lines, the use of as many as 20 process guns on a single production line may be required, with the accompanying multiplication of complexity as well as utility consumption. This limitation in process rate has restricted combustion wire thermal spray from high volume applications, thus a need exists to provide a means to increase the process rate of a single combustion wire gun.
Recent experimentation and disclosure of methods to preheat the wire have demonstrated a potential to increase the process rate considerably, but even higher process rates are needed to substantially decrease the number of guns needed for high volume applications. Since most thermal spray processes operate at very low efficiencies in terms of the energy supplied versus the energy required to perform the process there is demonstrated potential to increase process rate considerably.
Currently almost all thermal spray wires have a round cross section with an exception of some having been square. The round shape is actually the least appealing shape to use in a process where heat needs to be transferred through the surface as a round wire has the least exposed surface per unit volume and subsequently mass. Thus there is a theoretical and physical potential to improve combustion wire thermal spray through improvement of the feed stock wire shape.
A concern with regard to improving the process is the understanding that there is a need to maintain a seal between the wire and the bushing or sleeve used to guide the wire into the combustion region. If there is not a sufficient seal then there is a strong possibility that during shutdown the combustion gases will reverse back up the wire path and cause a backfire.
Another item to note is the general requirement that the cross sectional shape of the wire needs to coincide with the shape of the combustion flame such that the wire and combustion flame are concentric in order to ensure the wire is melted uniformly in the combustion flame. The use of wire guides and other means to present the wire concentrically to the flame have been incorporated in various forms since the initial invention of combustion flame spray guns.